Encapsulated hydrophilic polymers and their preparation

ABSTRACT

A multistaged polymer containing a hydrophilic core fully encapsulated with a hydrophobic shell without a tiecoat layer is disclosed. A method is also disclosed for encapsulating hydrophilic polymers including the steps of: 
     (1) emulsion polymerizing a hydrophilic core polymer from about 5% by weight to about 100% by weight, based on the total weight of the core polymer, of a hydrophilic monoethylenically unsaturated monomer and from 0% by weight to about 95% by weight, based on the total weight of the core polymer, of at least one nonionic monoethylenically unsaturated monomer; 
     (2) emulsion polymerizing, in the presence of the core polymer, at least one shell polymer from about 90% by weight to about 99.9% by weight, based on the total weight of shell polymer, of at least one nonionic monoethylenically unsaturated monomer and from about 0.1% by weight to about 10% by weight, based on the total weight of the shell polymer, of an acid-functional monoethylenically unsaturated monomer, 
     wherein the acid-functional monoethylenically unsaturated monomer is added to the polymerization of the shell polymer over 100% of the total shell monomer feed when the particle size of the core polymer is from about 130 nm to about 2.0 microns and over the first 50% of the total shell monomer feed when the particle size of the core polymer is less than about 130 nm. 
     This method of encapsulating a hydrophilic polymer with a hydrophobic polymer eliminates the need for a tiecoat layer.

This is a divisional of application Ser. No. 08/289,736, filed Aug. 12,1994 now U.S. Pat. No. 5,494,971.

FIELD OF THE INVENTION

This invention relates to multistaged polymer particles and theirpreparation and, more particularly, to multistaged polymer particles ofa hydrophilic polymer stage encapsulated with at least one hydrophobicpolymer stage.

BACKGROUND OF THE INVENTION

Multistaged polymers containing a hydrophilic core polymer encapsulatedby a hydrophobic shell polymer are known. It was, however, believed thata shell polymer of extremely non-polar or low-polar hydrophobic monomers("hydrophobic shell"), such as styrene, α-methyl styrene, vinyl toluene,ethylene, vinyl chloride and vinylidene chloride, could not be formeddirectly on a core containing a high level of hydrophilic monomers("hydrophilic core") without either:

(1) copolymerizing the extremely non-polar or low-polar hydrophobicmonomers with vinyl acetate, acrylonitrile or methacrylamide; or

(2) first forming a tiecoat on the hydrophilic core.

The tiecoat (referred to in some of the prior patents as "the firststage of sheath formation") was generally an acrylic polymer whichcompatibilized the hydrophilic core polymer with the one or morehydrophobic shell polymers, particularly for a hydrophilic core polymerhaving a particle size diameter of less than about 280 nanometers (nm).

SUMMARY OF THE INVENTION

This invention is directed to a multistaged polymer containing ahydrophilic core polymer fully encapsulated with a hydrophobic shellpolymer without the rise of a tiecoat layer. The hydrophilic corepolymer is formed from about 5% by weight to about 100% by weight, basedon the total weight of the core polymer, of a hydrophilicmonoethylenically unsaturated monomer and front 0% by weight to about95% by weight, based on the total weight of the core polymer, of atleast one nonionic monoethylenically unsaturated monomer. Thehydrophobic shell polymer formed from about 90% by weight to about 99.9%by weight, based on the total weight of shell polymer, of at least onenonionic monoethylenically unsaturated monomer and from about 0.1% byweight to about 10% by weight based on the total weight of the shellpolymer, of an acid-functional monoethylenically unsaturated monomer.The shell polymer fully encapsulates the core polymer and does notrequire a tiecoat layer.

This invention is also directed to a method for encapsulating ahydrophilic core polymer with a hydrophobic shell polymer including thesteps of:

(1) emulsion polymerizing a hydrophilic core polymer front about 5% byweight to about 100% by weight, based on the total weight of the corepolymer, of a hydrophilic monoethylenically unsaturated monomer and from0% by weight to about 95% by weight, based on the total weight of thecore polymer, of at least one nonionic monoethylenically unsaturatedmonomer;

(2) emulsion polymerizing, in the presence of the core polymer, at leastone shell polymer front about 90% by weight to about 99.9% by weight,based on the total weight of shell polymer, of at least one nonionicmonoethylenically unsaturated monomer and from about 0.1% by weight toabout 10% by weight, based on the total weight of the shell polymer, ofan acid-functional monoethylenically unsaturated monomer,

wherein the acid-functional monoethylenically unsaturated monomer isadded to the polymerization of the shell polymer over 100% of the totalshell monomer feed when the particle size of the core polymer is frontabout 130 nm to about 2.0 microns and over the first 50% of the totalshell monomer feed when the particle size of the core polymer is lessthan about 130 nm.

This method of encapsulating a hydrophilic polymer with a hydrophobicpolymer eliminates the need for a tiecoat layer.

DESCRIPTION OF THE INVENTION

The present invention involves a multistaged polymer containing ahydrophilic core polymer fully encapsulated with a hydrophobic shellpolymer and its method of preparation.

Description of Hydrophilic Core Polymer

The hydrophilic core polymer of the multistaged polymer of thisinvention is the product of emulsion polymerizing front about 5% byweight to about 100% by weight, based on the total weight of the corepolymer, of a hydrophilic monoethylenically unsaturated monomer and from0% by weight to about 95% by weight, based on the total weight of thecore polymer, of at least one nonionic monoethylenically unsaturatedmonomer.

Hydrophilic core polymers containing at least about 5% by weight, basedon the total weight of the core polymer, of at least one hydrophilicmonoethylenically unsaturated monomer have practical swellability forthe purposes of the present invention. There may be instances wherein,because of the hydrophobicity of certain comonomers or combinationsthereof in conjunction with the hydrophobic/hydrophilic balance of aparticular acid monomer, the copolymer may require less than 5% byweight, based on the total weight of the core polymer. Preferably, thelevel of hydrophilic monomer is from about 5% to about 100% by weight,based on the total weight of the core polymer; more preferably, fromabout 20% to about 60% by weight; and most preferably, from about 30% toabout 50% by weight. The hydrophilic core polymer may be made in asingle stage or step of the sequential polymerization or may be made bya plurality of steps in sequence.

This invention contemplates a hydrophilic core polymer wherein at leastone hydrophilic monoethylenically unsaturated monomer is polymerizedalone or with at least one nonionic monoethylenically unsaturatedmonomer. This process also contemplates, and includes in the term"hydrophilic monoethylenically unsaturated monomer," the use of anonpolymeric compound containing at least one carboxylic acid groupwhich absorbed into the core polymer before, during or after thepolymerization of the hydrophobic shell polymer as a replacement for thehydrophilic monoethylenically unsaturated monomer in the hydrophiliccore polymer, as described in U.S. Pat. No. 4,880,842. In addition, thisinvention contemplates, and includes in the term "hydrophilicmonoethylenically unsaturated monomer," the use of a latent hydrophiliccore polymer which contains no hydrophilic monoethylenically unsaturatedmonomer but which is swellable upon hydrolysis to a hydrophilic corepolymer as described in U.S. Pat. No. 5,157,084.

Suitable hydrophilic monoethylenically unsaturated monomer useful formaking the core polymer include monoethylenically unsaturated monomerscontaining acid-functionality such as monomers containing at least onecarboxylic acid group including acrylic acid and methacrylic acid,acryloxypropionic acid, (meth)acryloxypropionic acid, itaconic acid,aconitic acid, maleic acid or hydride, fumaric acid, crotonic acidmonomethyl maleate, monomethyl fumarate monomethyl itaconate and like.Acrylic acid and methacrylic acid are preferred.

Suitable nonpolymeric compounds containing at least one carboxylic acidgroup include C₆ -C₁₂ aliphatic or aromatic monocarboxylic acids anddicarboxylic acids, such as benzoic acid, m-toluic acid, p-chlorobenzoicacid, o-acetoxybenzoic acid, azelaic acid, sebacic acid, octanoic acid,cyclohexanecarboxylic acid, lauric acid and monobutyl phthalate and thelike.

Suitable nonionic monoethylenically unsaturated monomers for making thehydrophilic core polymer include styrene, α-methyl styrene,vinyltoluene, ethylene, vinyl acetate, vinyl chloride, vinylidenechloride, acrylonitrile, (meth)acrylamide, (C₁ -C₂₀) alkyl or (C₃ -C₂₀)alkenyl esters of (meth)acrylic acid, such as methyl (meth)acrylate,ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,benzyl (meth)acrylate, lauryl (meth)acrylate, oleyl (meth)acrylate,palmityl (meth)acrylate, stearyl (meth)acrylate and the like.

The hydrophilic core polymer, whether obtained by a single stage processor a process involving several stages, has an average size of about 50nm to about 2.0 micron, preferably 100 nm to 1.0 micron, more preferably200 nm to 500 nm diameter in unswollen condition. If the core isobtained from a seed polymer, the seed polymer may have an averageparticle size of about 30 nm to about 200 nm.

The hydrophilic core polymer may also optionally contain less than about20% by weight, based on the total weight of tile core polymer,preferably about 0.1% to about 3% by weight, of polyethylenicallyunsaturated monomer, wherein the amount used is generally approximatelydirectly proportional to the amount of hydrophilic monoethylenicallyunsaturated monomer used. Alternatively, the hydrophilic core polymermay contain from about 0.1% to about 60% by weight, based on the totalweight of the core polymer, of butadiene.

Suitable polyethylenically unsaturated monomers include comonomerscontaining at least two addition polymerizable vinylidene groups and areα,β-ethylenically unsaturated monocarboxylic acid esters of polyhydricalcohols containing 2-6 ester groups. Such comonomers include alkyleneglycol diacrylates and dimethacrylates, such as for example, ethyleneglycol diacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycoldiacrylate, 1,4-butylene glycol diacrylate propyleneglycol diacrylateand triethyleneglycol dimethylacrylate; 1,3-glycerol dimethacrylate;1,1,1-trimethylol propane dimethacrylate; 1,1,1-trimethylol ethanediacrylate; pentaerythritol trimethacrylate; 1,2,6-hexane triacrylate;sorbitol pentamethacrylate; methylene bis-acrylamide, methylenebis-methacrylamide, divinyl benzene, vinyl methacrylate, vinylcrotonate, vinyl acrylate, vinyl acetylene, trivinyl benzene, triallylcyanurate, divinyl acetylene, divinyl ethane, divinyl sulfide, divinylether, divinyl sulfone, diallyl cyanamide, ethylene glycol divinylether, diallyl phthalate, divinyl dimethyl silane, glycerol trivinylether, divinyl adipate; dicyclopentenyl (meth)acrylates;dicyclopentenyloxy (meth)acrylates; unsaturated esters of glycolmonodicyclopentenyl ethers; allyl esters of a,b-unsaturated mono- anddicarboxylic acids having terminal ethylenic unsaturation includingallyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate,diallyl itaconate and the like.

Description of Hydrophobic Shell Polymer

The hydrophobic shell polymer of the multistaged polymer of thisinvention is the product of emulsion polymerizing from about 90% byweight to about 99.9% by weight, based on the total weight of the shellpolymer, of at least one nonionic monoethylenically unsaturated monomerand from about 0.1% by weight to about 10% by weight, based on theweight of the shell polymer, of an acid-functional monoethylenicallyunsaturated monomer.

Suitable nonionic monoethylenically unsaturated monomers for making thehydrophobic shell polymer include styrene, co-methyl styrene,vinyltoluene, ethylene, vinyl acetate, vinyl chloride, vinylidenechloride, acrylonitrile, (meth)acrylamide, (C₁ -C₂₀) alkyl or (C₃ -C₂₀)alkenyl esters of (meth)acrylic acid, such as methyl (meth)acrylate,ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,benzyl (meth)acrylate, lauryl (meth)acrylate, oleyl (meth)acrylate,palmityl (meth)acrylate, stearyl (meth)acrylate and the like. Styrene ispreferred.

Suitable monoethylenically unsaturated monomers containingacid-functionality for making the hydrophobic polymer shell includeacrylic acid, methacrylic acid, acryloxypropionic acid,(meth)acryloxypropionic acid, itaconic acid, aconitic acid, maleic acid,maleic anhydride, fumaric acid, crotonic acid, monomethyl maleate,monomethyl fumarate, monomethyl itaconate and the like. Acrylic acid andmethacrylic acid are preferred.

Description of Polymerization Method

The method for encapsulating hydrophilic polymers includes the steps ofsequentially:

(1) emulsion polymerizing a hydrophilic core polymer; and

(2) emulsion polymerizing, in the presence of the hydrophilic corepolymer,

at least one hydrophobic shell polymer to completely encapsulate thecore. The crux of the invention is the manner and timing of the additionof the acid-functional monoethylenically unsaturated monomer in thepolymerization of the hydrophobic shell polymer. When the particle sizeof the core polymer is from about 130 nm to about 2.0 microns, theacid-functional monoethylenically unsaturated monomer used to make theshell may be added over 100% of the shell monomer feed, based on thetotal weight of the shell monomer feed, preferably over the first 50% ofthe feed, more preferably over the first 25% of the feed and mostpreferably over the first 10% of the feed. When the particle size of thecore polymer is less than about 130 nm, the acid-functionalmonoethylenically unsaturated monomer may be added over the first 50% ofthe feed, more preferably over the first 25% of the feed and mostpreferably over the first 10% of the feed.

As used herein, the term "sequentially emulsion polymerized" or"sequentially emulsion produced" refers to polymers (includinghomopolymers and copolymers) which are prepared in aqueous medium by anemulsion polymerization process wherein the dispersed polymer particlesof a preformed latex or "seed" polymer in the aqueous medium areincreased in size by deposition thereon of polymerized product of one ormore successive monomer charges introduced into the medium containingdispersed particles of the preformed latex in one or more subsequentstages.

In the sequential emulsion polymerization with which the presentinvention is concerned, the term "seed" polymer is used to refer to anaqueous emulsion polymer dispersion which may be the initially-formeddispersion, that is, the product of a single stage of emulsionpolymerization or it may be the emulsion polymer dispersion obtained atthe end of any subsequent stage except the final stage of the sequentialpolymerization. Thus, a hydrophilic core polymer which is hereinintended to be fully encapsulated with a shell by one or more subsequentstages of emulsion polymerization may itself be termed a seed polymerfor the next stage wherein the shell-forming polymer is deposited onsuch seed polymer particles.

The method of this invention contemplates that the hydrophobic shellpolymer may be made in a single stage or step of the sequentialpolymerization or may be made by a plurality of steps in sequencefollowing the polymerization of hydrophilic core polymer without theneed for a tiecoat layer. The first stage of emulsion polymerization inthe process of the present invention may be the preparation of a seedpolymer containing small dispersed polymer particles insoluble in theaqueous emulsion polymerization medium. This seed polymer may or may notcontain any hydrophilic monomer component but provides particles ofminute size which form the nuclei on which the hydrophilic core polymer,with or without nonionic comonomer, is formed.

A water-soluble free radical initiator is utilized in the aqueousemulsion polymerization. Suitable water-soluble free radical initiatorsinclude hydrogen peroxide; tert-butyl peroxide; alkali metal such assodium, potassium and lithium persulfate; ammonium persulfate; andmixture of such an initiator with a reducing agent, such as a sulfite,including an alkali metal metabisulfite, hydrosulfite, and hyposulfite;sodium formaldehyde sulfoxylate; and a reducing sugar, to form a redoxsystem. The amount of initiator may be from 0.01% by weight to about 2%by weight of the monomer charged and in a redox system, a correspondingrange of 0.01% by weight to about 2% by weight of reducing agent may beused. The temperature may be in the range of about 10° C. to 100° C. Inthe case of the persulfate systems, the temperature is preferably in therange of 60° C. to 90° C. In the redox system, the temperature ispreferably in the range of 30° C. to 70° C., preferably below about 60°C., more preferably in the range of 30° C. to 45° C. The proportion ofemulsifier may be zero, in the situation wherein a persulfate initiatoris used, to about 0.75% by weight, based on the weight of total weightof the core polymer. By carrying out the emulsion polymerization whilemaintaining low levels of emulsifier, the subsequent stages ofpolymer-formation deposit the most-recently formed polymer on theexisting dispersed polymer particles resulting from the preceding stepor stage. As a general rule, the amount of emulsifier should be keptbelow that corresponding to the critical micelle concentration for aparticular monomer system, but while this limitation is preferable andproduces a unimodal product, it has been found that in some systems thecritical micelle concentration of the emulsifier may be exceededsomewhat without the formation of an objectionable or excessive numberof dispersed micelles or particles. It is for the purpose of controllingthe number of micelles during the various stages of polymerization sothat the deposition of the subsequently formed in the polymer in eachstage occurs upon the dispersed micelles or particles formed in theprevious stages, that the concentration of emulsifier is kept low.

Any nonionic or anionic emulsifier may be used, either alone ortogether. Examples of suitable nonionic type of emulsifier includetert-octylphenoxyethylpoly(39)-ethoxyethanol, andnonylphenoxyethylpoly-(40)ethoxyethanol. Examples of suitable anionicemulsifiers include sodium lauryl sulfate, sodiumdodecylbenzenesulfonate, and tert-octylphenoxyethoxypoly(39)ethoxyethylsulfate, sodium salt. The viscosity-average molecular weight of thepolymer formed in a given stage may range from 100,000, or lower if achain transfer agent is used, to several million molecular weight. When0.1% by weight to 20% by weight, based on the weight of the monomer, ofa polyethylenically unsaturated monomer mentioned hereinbefore is usedin making the acid polymer, the molecular weight is increased whether ornot crosslinking occurs. The use of the polyethylenically unsaturatedmonomer reduces the tendency of the core polymer to dissolve when themultistaged polymer is treated with a swellant for the core. If it isdesired to produce a hydrophilic core polymer having a molecular weightin the lower part of the range, such as from 500,000 down to as low asabout 20,000, it is frequently most practical to do so by avoiding thepolyethylenically unsaturated monomers and using a chain transfer agentinstead, such as 0.05% to 2% or more thereof, examples being alkylmercaptans, such as sec-butyl mercaptan.

The polymerization of the shell polymer may be performed in the samereaction vessel in which the formation of the core was accomplished orthe reaction medium containing the dispersed core particles may betransferred to another reaction container. It is generally unnecessaryto add emulsifier unless a polymodal product is desired, but in certainmonomer/emulsifier systems for forming the shell, the tendency toproduce gum or coagulum in the reaction medium may be reduced orprevented by the addition of about 0.05% to about 2.0% by weight, basedon total weight of the shell polymer, of emulsifier without detriment tothe deposition of the polymer formed on the previously formed coreparticles.

The amount of polymer deposited to form shell polymer is generally suchas to provide an overall size of the multistage polymer particle ofabout 70 nm to about 4.5 microns, preferably about 100 nm to about 3.5microns, more preferably about 200 nm to about 2.0 microns, in unswollencondition (that is, before any neutralization to raise the pH to about 6or higher) whether the shell polymer is formed in a single stage or in aplurality of stages. In unswollen state, the ratio of core weight to thetotal weight on average is from 1:4 to 1:100.

Preferred Embodiment

The method of the present invention may be extended to form voidedpolymer particles by adding to the hydrophilic core polymer fullyencapsulated with a hydrophobic shell polymer, a suitable swelling agentto which the hydrophobic shell polymer is permeable.

In a preferred embodiment, voided polymer particles may be formed byswelling the core polymer with a suitable conjugate base and a solvent,when necessary, which permeates through the shell polymer and thendrying the swollen multistaged polymer particles. The voided polymerparticles produced by the method of this invention impart improvedgloss, brightness and opacity to paper coating formulations to whichthey are added.

The monomers used and the relative proportions thereof in anyhydrophobic shell polymer formed should be such that it is permeable toan aqueous or gaseous volatile or fixed basic swellant for thehydrophilic core polymer. Monomeric mixtures for making the hydrophobicshell polymer contain from about 0.1% by weight to about 10% by weight,based on the total weight of the shell polymer, of an acid-functionalmonoethylenically unsaturated monomer. However, the proportion ofacid-functional monoethylenically unsaturated monomer in the shellpolymer should not exceed one-third the proportion thereof in the corepolymer. The content of acid-functional monoethylenically unsaturatedmonomer in the shell polymer may serve several functions:

(1) stabilizing of the final sequential polymer dispersion;

(2) assuring permeability of the hydrophobic shell polymer to a swellantfor the hydrophilic core polymer; and

(3) compatibilizing the hydrophobic shell polymer with the hydrophiliccore polymer so that the core may be fully encapsulated with shell.

The hydrophilic core polymer of the multistage polymer particle isswollen when the polymer particles are subjected to a basic swellingagent that permeates the shell to at least partially neutralize (to a pHof at least about 6 to 10) the hydrophilic-functionality of thehydrophilic core polymer and thereby to cause swelling by hydration ofthe hydrophilic core polymer. The expansion may involve partial mergingof the outer periphery of the core into the pores of the inner peripheryof the shell and also partial enlargment or bulging of the shell and theentire particle overall. When the swelling agents removed by drying, theshrinkage of the core tends to develop a void, the extent of whichdepends upon the resistance of the shell to restoration to its previoussize.

Suitable swelling agents for hydrophilic core polymer include volatilebases such as ammonia, ammonium hydroxide, and volatile lower aliphaticamines, such as morpholine, trimethylamine, and triethylamine, and thelike; fixed or permanent bases such as potassium hydroxide, lithiumhydroxide, zinc ammonium complex, copper ammonium complex, silverammonium complex, strontium hydroxide, barium hydroxide and the like.Solvents, such as, for example, ethanol, hexanol, octanol, Texanol®solvent and those described in U.S. Pat. No. 4,594,363, may be added toaid in fixed or permanent base penetration.

The voided latex particles produced by the method of the presentinvention are useful in aqueous coating compositions, such asaqueous-based paint and paper coatings. The voided polymer particlesproduced by the method of this invention impart improved gloss,brightness and opacity to paper coating formulations to which they areadded. Also, the voided polymer particles produced by the method of thisinvention impart opacity to aqueous coating compositions, such aspaints, to which they are added.

When the hydrophilic core polymer is fully encapsulated, it does nottitrate with alkali metal bases under normal analytical conditions ofabout 1 hour and at room temperature. To demonstrate full encapsulationin the illustrative examples, samples were removed during the course ofthe shell polymerization and titrated with sodium hydroxide. The amountof shell monomer feed varied to fully encapsulate the hydrophilic corepolymers. It is desirable to have lower amounts of shell monomer toachieve full encapsulation.

The following examples illustrate specific aspects and particularembodiments of the invention which, however, in not to be construed aslimited thereby.

EXAMPLE 0

Synthesis of Hydrophilic Core Polymer (40% Hydrophilic MonoethylenicallyUnsaturated Monomer)

(a) A 5-liter, four necked, round bottom flask is equipped with paddlestirrer, thermometer, nitrogen inlet and reflux condenser. Deionizedwater, 1700 grams, is added to the kettle and heated to 85° C. under anitrogen atmosphere A monomer emulsion consisting of 335 grams ofdeionized water, 3.5 grams of sodium dodecylbenzenesulfonate (SDS, 23 ),4.35 grams of methacrylic acid, and 364.5 grams of methyl methacrylateis prepared. A portion of this monomer emulsion, 82 grams, is added tothe heated kettle. After the removal of the 82 grams of monomeremulsion, 7 grams of SDS and 241 grams of methacrylic acid are added tothe remaining monomer emulsion. After stirring the monomer emulsionkettle charge for five minutes at 800° C. under nitrogen, a solution of2.75 grams of sodium persulfate in 15 grams of deionized water is addedto the kettle. A 1° to 2° C. reaction exotherm occurs, the reactionmixture is stirred for 10 minutes. The remaining monomer emulsion isthen added to the kettle over a 2 hour period at 80° C. After thecompletion of the monomer feed, the dispersion is held at 80° C. for 20minutes, cooled to 25° C. and filtered to remove any coagulum formed.The filtered dispersion has a pH of 3.11, 22.27% solids content and anaverage particle size diameter of 330 nm.

(b) The process in core synthesis (a) was repeated except the SDS levelin the monomer emulsion was increased to 12 grams from 3.5 grams and theamount of SDS added to the monomer emulsion after removal of the monomerpreform was raised to 14 grams from 7 grams. The filtered dispersion hasa pH of 3.16, a 22.3% solids content and an average particle size of 208nm.

(c) A 5-liter, four necked, round bottom flask is equipped with paddlestirrer, thermometer, nitrogen inlet and reflux condenser. Deionizedwater, 1700 grams, is added to the kettle and heated to 85° C. under anitrogen atmosphere. A monomer emulsion consisting of 335 grams ofdeionized water, 20.0 g of sodium dodecyl benzene sulfonate (SDS, 23%),4.35 grams of methacrylic acid, and 364.5 grams of methyl methacrylateis prepared. A portion of this monomer emulsion, 100 grams, is added tothe heated kettle. After the removal of the 100 grams of monomeremulsion, 22 grams of SDS and 241 grams of methacrylic acid are added tothe remaining monomer emulsion. After stirring the monomer emulsionkettle charge for five minutes at 80° C. under nitrogen, a solution of2.75 grams of sodium persulfate in 15 grams of deionized water is addedto the kettle. A 1° to 2° C. reaction exotherm occurs, the reactionmixture is stirred for 10 minutes. The remaining monomer emulsion isthen added to the kettle over a 2-hour period at 80° C. After thecompletion of the monomer feed, the dispersion is held at 80° C. for 20minutes, cooled to 25° C. and filtered to remove any coagulum formed.The filtered dispersion has pH of 3.11, 22.28% solids content and anaverage particle size of 132 nm.

EXAMPLE 1

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over first 2.7% Shell Monomer Feed (Core Particle Size=330 nm)

A 5-liter round-bottomed flask is equipped with paddle stirrer,thermometer, nitrogen inlet and reflux condenser. To 2070 grams ofdeionized water heated to 90° C. in the flask under a nitrogenatmosphere there is added 1.6 grams of sodium persulfate dissolved in 25grams of deionized water. This is immediately followed by 383.58 gramsof the core prepared in Example 0-(a). A monomer emulsion consisting of400 grams of deionized water, 4.5 grams of SDS and 1080 grams of styreneis added to the kettle at the rate of 4 grams/minute. A separate feed of12.5 grams of acrylic acid in 38.5 grams of deionized water is startedat the same time as the monomer emulsion feed, at the rate of 5grams/minute. Along with these feeds a solution of 2.77 grams of sodiumpersulfate in 130 grams of deionized water is co-fed to the kettle atthe rate of 1.3 grams/minute. The reaction mixture is held at 80° C. andafter 10 minutes the acrylic acid cofeed is complete and the monomeremulsion feed rate is increased to 8 grams/minutes. Twenty minutes laterthe monomer emulsion feed rate is increased again to 19.5 grams/minuteand the reaction temperature is allowed to increase to 90° C. After 1157grams of the monomer emulsion has been fed to the reaction mixture, asolution of 42.0 grams of ammonium hydroxide (28%) in 72 grams ofdeionized water is added to the kettle. After all of the monomeremulsion and catalyst cofeed have been added, the reaction mixture isheld at 90° C. for 10 minutes. This is followed by a cofeed of 2 gramsof sodium persulfate in 100 grams of deionized water over a 15 minuteperiod. The reaction mixture is then cooled to room temperature andfiltered to remove any coagulum formed. The final latex product had a27.23% solids content and pH of 9.7. Diluted latex was dried on amicroscope slide and immersed in hydrocarbon oil (n_(D) =1.51) andexamined with an optical microscope at 1000×. A single air void can beobserved inside of each particle as a dark circle. The swollen particlewas incorporated into a film to measure the Kubelka-Munk scatteringcoefficient (S/mil) as described in U.S. Pat. No. 4,427,836. S/mil ofthe resultant film was 0.45. Samples of either high acid core becomescompletely soluble when neutralized with sufficient base. But, whentotally covered by the polymer sheath, hard bases such as sodiumhydroxide will not neutralize the core at room temperature. Sodiumhydroxide titration on samples removed during the course of thisreaction demonstrated that complete core encapsulation occurred afterthe polymerization of 400 grams of monomer emulsion onto the core.

EXAMPLE 2

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over 100% Shell Monomer Feed (Core Particle Size=330 nm)

The reaction in example 1 was repeated, except that the acrylic acid wasadded to the monomer emulsion. The resulting latex had a 27.23% solidscontent, pH of 9.7 and a S/mil of 0.28. Voids were observed in the drypolymer particles through examination via the microscope methoddescribed in Example 1. Titration of in-process samples demonstratedcomplete core encapsulation after the polymerization of 1000 grams ofmonomer emulsion onto the core.

EXAMPLE 3

Acid-functional Monoethylenically Unsaturated Monomer (Methacrylic Acid)added over first 2.7% Shell Monomer Feed (Core Particle Size=330 nm)

The reaction in Example 1 was repeated, except that methacrylic acid (15grams) was used in place of the acrylic acid. The resulting latex had a27.04% solids content, pH of 9.6 and a S/mil of 0.3. Voids were observedin the dry polymer particles through examination via the microscopemethod described in Example 1. Titration of in-process samplesdemonstrated complete core encapsulation after the polymerization of 400grams of monomer emulsion onto the core.

EXAMPLE 4

Acid-functional Monoethylenically Unsaturated Monomer (Methacrylic Acid)added over 100% Shell Monomer Feed (Core Particle Size=330 nm)

Example 3 was repeated except the methacrylic acid was added to themonomer emulsion. The reaction was thicker at the end of feeds and 300grams of extra water was added. The resulting latex had a 25.59% solidscontent, pH of 9.7 and a S/mil of 0.2. Voids were observed in the drypolymer particles through examination via the microscope methoddescribed in Example 1. Titration of in-process samples demonstratedcomplete core encapsulation after the polymerization of 800 grams ofmonomer emulsion onto the core.

EXAMPLE 5

Fixed Base Swellant

Example 1 was repeated except 60 grams of diethanolamine was used inplace of the ammonia. The resulting latex had a 27.81% solids content,pH of 8.7 and a S/mil of 0.2. Voids were observed in the dry polymerparticles through examination via the microscope method described inExample 1. Titration of in-process samples demonstrated complete coreencapsulation after the polymerization of 400 grams of monomer emulsiononto the core.

EXAMPLE 6

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over first 1.6% Shell Monomer Feed (Core Particle Size=208 nm andAcid Feed Time=10 minutes)

A 5-liter round-bottomed flask is equipped with paddle stirrer,thermometer, nitrogen inlet and reflux condenser. To 1192 grams ofdeionized water heated to 81° C. in the flask under a nitrogenatmosphere there is added 3.6 grams of sodium persulfate dissolved in 60grams of deionized water. This is immediately followed by 142.6 grams ofthe core prepared in Example 0-(b). A monomer emulsion consisting of 723grams of deionized water, 8.04 grams of SDS and 1710.6 grams of styreneis added to the kettle at the rate of 4 grams/minutes. A separate feedof 17 grams of acrylic acid in 33 grams of deionized water is started atthe same time as the monomer emulsion feed, at the rate of 5grams/minutes. The reaction mixture is held at 80° C. and after 10minutes the acrylic acid cofeed is complete and the monomer emulsionfeed rate is increased to 8 grams/minutes. Ten minutes later the monomeremulsion feed rate is increased again to 12 grams/minutes for twentyminutes. The monomer emulsion feed rate is increased to 18 grams/minutesand a solution of 2 grams of sodium persulfate in 85 grams of deionizedwater is co-fed to the kettle at the rate of 0.7 grams/min. The reactiontemperature is allowed to increase to 90° C. After all of the monomeremulsion has been fed to the reaction mixture, a solution of 19.0 gramsof ammonium hydroxide (28%) in 20 grams of deionized water is added tothe kettle and the reaction mixture is held at 90° C. for twentyminutes. The reaction is cooled to 80° C. and a mixture of 1 grams ofsodium persulfate in 30 grams of deionized water is added to the kettle.This is followed by a solution of 1.5 grams of 1% versene and 15 gramsof 0.1% iron sulfate and a solution of 1.5 grams isoascorbic acid in 30grams of deionized water. The reaction mixture is held at 80° C. for 10minutes and then cooled to room temperature and filtered to remove anycoagulum formed. The final latex product had a 43% solids content, a pHof 8.0, a S/mil. of 0.18 and a particle size of 0.82 micron. Voids wereobserved in the dry polymer particles through examination via themicroscope method described in Example 1. Titration of in-processsamples demonstrated complete core encapsulation after thepolymerization 800 grams of monomer emulsion onto the core.

EXAMPLE 7

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over first 1.6% Shell Feed (Core Particle Size=208 nm and AcidFeed Time=0 minutes)

A 5-liter round-bottomed flask was equipped with paddle stirrer,thermometer, nitrogen inlet and reflux condenser. To 1192 grants ofdeionized water heated to 81° C. in the flask under a nitrogenatmosphere there was added 3.6 grams of sodium persulfate dissolved in60 grams of deionized water. This was immediately followed by 142.6grams of the core prepared in Example 0-(b). To the kettle was added 40grams of a monomer emulsion made up of 723 grams of deionized water,8.04 grams of SDS and 1710.6 grams of styrene. A separate charge of 17grams of acrylic acid in 33 grams of deionized water was added to thekettle. The remaining monomer emulsion was fed to the kettle at a rateof 4 grams/minute. The reaction mixture was held at 80° C. and after 10minutes the monomer emulsion feed rate was the increased to 8grams/minutes. Ten minutes later the monomer emulsion feed rate wasincreased to 12 grams/minutes for twenty minutes. The monomer emulsionfeed rate was increased to 18 grams/minutes and a solution of 2 grams ofsodium persulfate in 85 grams of deionized is co-fed to the kettle atthe rate of 0.7 grams/minute. The reaction temperature was allowed toincrease to 90° C. After all of the monomer emulsion had been fed to thereaction mixture, a solution of 19.0 grams of ammonium hydroxide (28%)in 20 grams of deionized was added to the kettle and the reactionmixture was held at 90° C. for twenty minutes. The reaction was cooledto 80° C. and a mixture of 1 gram of sodium persulfate in 30 grams ofdeionized water was added to the kettle. This was followed by a solutionof 1.5 grams of 1% versene and 15 grams of 0.1% iron sulfate and asolution of 1.5 grams of isoascorbic acid in 30 grams of deionizedwater. The reaction mixture was held at 80° C. for 10 minutes and thencooled to room temperature and filtered to remove any coagulum formed.The final latex product had a 43% solids content, a pH of 8.0 and aparticle size of 0.82 microns. Voids were observed in the dry polymerparticles through examination via the microscope method described inExample 1. Titration of in-process samples demonstrated complete coreencapsulation after the polymerization of 400 grams of monomer emulsiononto the core.

EXAMPLE 8

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over 100% Shell Monomer Feed (Core Particle Size=208 nm)

The process in Example 6 was repeated except the acrylic acid was addedto the styrene monomer emulsion. The final latex product had a 43%solids content, a pH of 8.4, a S/mil of 0.165 and a particle size of0.82 micron. Voids were observed in the dry polymer particles throughexamination via the microscope method described in Example 1. Titrationof in-process samples demonstrated complete core encapsulation after thepolymerization of 2400 grams of monomer emulsion onto the core.

EXAMPLE 9

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over first 1.6% Shell Monomer Feed (Core Particle Size=132 nm)

The process in Example 6 was repeated except 142 grams of core fromexample 0-c was utilized. The final latex product had a 43% solidscontent, a pH of 7.9, a S/mil of 0.11 and a particle size of 0.53micron. Voids were observed in the dry polymer particles throughexamination via the microscope method described in Example 1. Titrationof inprocess samples demonstrated complete core encapsulation after thepolymerization of 1000 grams of monomer emulsion onto the core.

EXAMPLE 10

Acid-functional Monoethylenically Unsaturated Monomer (Acrylic Acid)added over 100% Shell Monomer Feed (Core Particle Size=132nm)--Comparative Example

The process in Example 8 was repeated except 142 grams of core fromExample 0-(c) was utilized. After 800 grams of monomer emulsion wasadded the reaction mixture started to thicken. The reaction was stoppeddue to massive levels of coagulum. The remaining 1600 grams of monomeremulsion could not be polymerized. This comparative example demonstrateda lack of encapsulation.

EXAMPLE 11

Synthesis of Pre-Core Polymer

A 3-liter, round-bottomed flask with 4 necks was fitted with refluxcondenser, paddle stirrer, thermometer and nitrogen inlet. 1500 grams ofdeionized water were added to the flask and stirred under a nitrogenatmosphere at 85° C. To the deionized water were added 3 grams of sodiumpersulfate and 40 grams of an acrylic seed latex having a 46.5% solidscontent and an average diameter of 95 nanometers. A monomer emulsion(140 grams of water, 6 grams of 23% SDS, 360 grams of isobutylmethacrylate and 40 grams of n-dodecyl mercaptan) was added over 2 hoursalong with 3 grams of sodium persulfate dissolved in 80 grams of water.The resultant latex was held at 85° C. for 30 minutes, cooled andfiltered. The resultant polymer latex had an 18.7% solid content, and aaverage particle diameter of 260 nm.

EXAMPLE 12

Encapsulation of Core containing Nonpolymeric Acid

A 5-liter round-bottomed flask is equipped with paddle stirrer,thermometer, nitrogen inlet and reflux condenser. A mixture of 1134grams deionized water and 51.66 g of benzoic acid is heated to 81° C. inthe flask under a nitrogen atmosphere. To this mixture is added 129grams of the pre-core polymer from example 11. After stirring at 81° C.for 15 minutes a solution of 1.5 grams of sodium persulfate in 75 gramsof water is added to the kettle. A monomer emulsion consisting of 713grams of deionized water, 6.03 grams of SDS, 2.18 grams of linseed oilfatty acid and 1283 grams of styrene is added to the kettle at the rateof 3 grams/minute. A separate feed of 12.75 grams of acrylic acid in24.75 grams of deionized water is started at the same time as themonomer emulsion feed, at the rate of 3.8 grams/minute. A catalystcofeed of 2.7 grams sodium persulfate in 112.5 grams of water is startedat a rate of 0.7 grams/minute. The reaction mixture is held at 80° C.and after 10 minutes the acrylic acid cofeed is complete and the monomeremulsion feed rate is increased to 6 grams/minute. Ten minutes later themonomer emulsion feed rate is increased again to 9 grams/minute fortwenty minutes. The monomer emulsion feed rate is increased to 13.5grams/minute until all the monomer emulsion is added to the kettle.After the addition of 1000 grams of monomer emulsion the reactiontemperature is allowed to increase to 90° C. At the end of thepolymerization, a solution of 37.5 grams of ammonium hydroxide (28%) in300 grams of deionized water is added to the kettle and the reactionmixture is held at 90° C. for twenty minutes. A mixture of 1 grams ofsodium persulfate in 30 grams of deionized water is added to the kettle.This is followed by a solution of 1.5 grams of 1% versene and 15 gramsof 0.1% iron sulfate and a solution of 1.5 grams isoascorbic acid in 30grams of deionized water. The reaction mixture is held at 90° C. for 10minutes and then cooled to room temperature and filtered to remove anycoagulum formed. The reaction was found to be high in coagulum andresidual styrene, but examination of the dried particles according tothe method in Example 1 indicated the presence of air voids.

EXAMPLE 3

Encapsulation of Cores containing no Hydrophilic MonoethylenicallyUnsaturated Monomer Core/Formation of Voids via Hydrolysis of Core

A 5-liter round-bottomed flask is equipped with paddle stirrer,thermometer, nitrogen inlet and reflux condenser. To 1192 grams ofdeionized water heated to 81° C. in the flask under a nitrogenatmosphere there is added 3.6 grams of sodium persulfate dissolved in 60grams of deionized water. This is immediately followed by 103.89 gramsof the core prepared according to the example in U.S. Pat. No. 5,157,084assigned to Dow Chemical. A monomer emulsion consisting of 723 grams ofdeionized water, 8.04 grams of SDS, 2.18 grams of linseed oil fatty acidand 1710.6 grams of styrene is added to the kettle at the rate of 4grams/minute. A separate feed of 17 grams of acrylic acid in 33 grams ofdeionized water is started at the same time as the monomer emulsionfeed, at the rate of 5 grams/minute. The reaction mixture is held at 80°C. and after 10 minutes the acrylic acid cofeed is complete and themonomer emulsion feed rate is increased to 8 grams/minute. Ten minuteslater the monomer emulsion feed rate is increased again to 12grams/minute for twenty minutes. The monomer emulsion feed rate isincreased to 18 grams/minute and a solution of 2 grams of sodiumpersulfate in 85 grams of deionized water is co-fed to the kettle at therate of 0.7 grams/minute. The reaction temperature is allowed toincrease to 90° C. After all of the monomer emulsion has been fed to thereaction mixture, a portion of the resulting latex is neutralized withammonia to pH 11 and heated to 150° C. in a stainless steel reactor.Examination of dried samples of this reaction mixture according to themethod in Example 1 indicated the presence of air voids.

I claim:
 1. A multistaged polymer, comprising:(a) a hydrophilic corepolymer formed from about 5% by weight to about 100% by weight, based onthe total weight of the core polymer, of a hydrophilic monoethylenicallyunsaturated monomer and from 0% by weight to about 95% by weight, basedon the total weight of the core polymer, of at least one nonionicmonoethylenically unsaturated monomer; and (b) a hydrophobic shellpolymer formed from about 90% by weight to about 99.9% by weight, basedon the total weight of shell polymer, of styrene and from about 0.1% byweight to about 10% by weight, based on the total weight of the shellpolymer, of an acid-functional monoethylenically unsaturated monomer,wherein said shell polymer fully encapsulates said core polymer.
 2. Themultistaged polymer of claim 1 wherein said acid-functionalmonoethylenically unsaturated monomer is a monomer selected from thegroup consisting of acrylic acid, methacrylic acid, acryloxypropionicacid, methacryloxypropionic acid, acryloxyacetic acid,methacryloxyacetic acid, crotonic acid, itaconic acid, aconitic acid,maleic acid, fumaric acid, monomethyl maleate, monomethyl itaconate,monomethyl fumarate and mixtures thereof.